Interleukin-5 (IL-5), a cytokine involved in eosinophil chemotaxis and activation, is felt to play an important role in lung eosinophil influx in asthma. As we have shown that eosinophil influx into the lung increases at night in nocturnal asthma (NA), we hypothesized that expression of IL-5 messenger RNA (mRNA) would be increased at night in subjects with NA. Methods: In an ongoing study, six subjects with NA, defined as an overnight fall in peak expiratory flow rate of > 15%, and 4 non-asthmatic controls underwent bronchoscopy with endobronchial biopsy at 4 pm and 4 am in a random order separated by one week. Expression of IL-5 mRNA was evaluated employing in situ hybridization using 35S labeled antisense and sense riboprobes. The number of epithelial and interstitial cells positive for IL-5 mRNA was determined by an individual blinded to asthma status. The number of positive epithelial cells was calculated as a percentage of the total number, and the number of positive interstitial cells was expressed as number/mm2. Results: IL-5 mRNA was expressed by epithelial and interstitial cells in both asthmatics and controls. There were no significant differences between the two groups in the number of IL-5 mRNA positive cells in both epithelial and interstitial cells at either time point. Within the NA group, the percentage of epithelial cells expressing IL-5 mRNA was higher at 4 am than 4 pm and this difference trended toward significance (12.4 1 3.2 vs. 5.2 1 2.6% at 4 am and 4 pm, p=0.06). Conclusions: Expression of IL-5 mRNA appears to occur in both large airway epithelial and interstitial cells of asthmatic and control subjects. In addition, expression of IL-5 mRNA trends toward increased expression at night in NA. Examination of IL-5 protein expression by epithelial cells in further studies with more subjects is necessary to confirm these present observations.